The other answer here is for Bradgate not Beacon. There are various walks and paths you can choose so the length of walk and choice of inclines is up to you really. There are 2 car parks with paths off each so that should give an idea of how big it is but there are short rounds of 20 mins or so. Lo get walks up to hour and half depending on your speed etc.
Get quick answers from Beacon Hill Country Park staff and past visitors.